A generally license free frequency band called the industrial-scientific-medical (ISM) band has emerged for short range low power applications, such as industrial controls, telemetry and low power data transmission. A range of frequencies from 300 MHz to 1 Ghz is generally provided for ISM. For example, the European ISM standard provides for operation at 433 MHz and at frequencies ranging from 800 MHz to 930 MHz. The US ISM standard provides for a band at 300 MHz and in a range from 902 MHz to 928 MHz. An ISM transmitter may, therefore, need to operate at more than one carrier frequency. The transmitter's carrier frequency fc is typically determined by a reference frequency fref from a reference crystal. A range of carrier frequency values, e.g. ranging from 800 MHz to 930 MHz, can be achieved by synthesizing the carrier frequency from the reference frequency to obtain the carrier frequency. For example, a phased-lock-loop (PLL) synthesizer may be used to synthesize the carrier frequency from the reference frequency.
Examples of applications for ISM transmitters include: security alarms, telemetry, environment control systems, wireless data repeaters, personal/patient data logging, access and movement monitoring, remote metering, barcode readers, wireless keyboard and mouse, remote keyless entry, remote tire pressure control, garage door openers, and doorbells.
U.S. Pat. No. 6,253,068 issued on Jun. 26, 2001 to Elder et al. discloses an example of a fully integrated all-CMOS AM transmitter with automatic antenna tuning. In the disclosed system, the antenna serves as a resonator for the oscillator block within the phase-lock-loop (PLL). By using the antenna as the resonator, Elder et al. automatically provides that a tuning varactor in the resonant circuitry to receive the proper tuning voltage in order to tune the antenna (VCO resonator) to the desired frequency. However, since continuous tuning is necessary for the PLL, only varactors can be used, which require higher voltage levels that place a relatively high minimum limit on the supply voltage and the signal amplitude on the antenna. Further, any mistuning effects that may occur due to the antenna may cause the oscillator to fall out of the tuning range of the PLL thereby disrupting oscillation and causing the circuit to cease functioning.
U.S. Pat. Nos. 5,136,719 and 5,483,688 describe an approach for antenna tuning that works for small signals that are suitable for use in receiver circuits rather than transmitter circuits. The tuning elements used in these patents are varactors, which require a relatively high minimum supply voltage because a decoupling capacitor is required in order to control the tuning elements.
U.S. Pat. No. 5,136,719 issued on Aug. 4, 1992 to Gaskill et al. discloses another automatic antenna tuning method and apparatus. In Gaskill et al. system, an antenna receives radio frequency signals in a desired reception band from 88 to 108 MHz. The antenna is automatically tuned to receive packets of information on a periodic basis. During a tuning mode, the control circuit sweeps a varactor biasing voltage over its full range to measure an optimum level. To determine the optimum tuning condition, a control circuit receives from a receiver subsystem a Received Signal Strength Indicator (RSSI) signal that is indicative of received signal strength. Once the sweep is concluded, the system sets the tuning element, and hence the antenna, to the value that produced the maximum RSSI signal. A packet of information is then received and passed to a protocol decoder to decode the information. The method of Gaskill et al., however, the antenna tuning approach lacks an adaptive capability with respect to changes in the device environment or in component selection. Further, overall system requirements typically impose certain time limitations on the time available for the antenna tuning procedure, which therefore impose limitations on the accuracy of the antenna tuning procedure as set forth under the Gaskill method.
U.S. Pat. No. 5,483,688 issued on Jan. 9, 1996 to English et al. discloses still another method and apparatus for automatically tuning an adaptive antenna. A predictor value is used to establish an antenna tuning voltage sub-range that is most likely to contain the optimum antenna tuning voltage, where the sub-range is a smaller range than the full antenna tuning voltage range. The antenna tuning voltage sub-range is then traversed while monitoring a signal strength indicator to identify an antenna tuning voltage providing optimum tuning conditions, which improves the execution time and accuracy of the antenna tuning.
Another approach to antenna tuning is found in the ASCell3913 868 MHz, 433 MHz and 315 MHz ISM Band FSK Transmitter from Austria Micro Systems (AMS). (See the ASCell3913 Preliminary Data Sheet, Rev. No. D5, March 2002, incorporated by reference in its entirety.) The ASCell3913 solution uses three capacitance values to tune an antenna. Antenna tuning is allowed for only a short period when the transmitter is powered-up. The resulting state of the tuning circuit is maintained for the duration of the transmission session. This solution has a limited accuracy due to the reduced number of capacitance cells and cannot follow changes in conditions that may occur during long transmission periods.